rs1142345 is a SNP in the TPMT gene, potentially encoding a variant incapable of detoxifying byproducts of certain antineoplastic and immunosuppressant drugs. In general, individuals must have two nonfunctioning TPMT alleles for the toxicity to be pronounced.

The risk allele for this SNP is rs1142345(G), and when it is the only variant in the TPMT gene, it encodes the TPMT*3C allele. While still rare, it is more common in African-Americans (2.4% of all alleles) than in Caucasians. Note that if the same allele also carries the rs1800460(A) SNP, the allele is actually a TPMT*3A allele (OMIM).

The medicine 6-Mercaptopurine is metabolized by TPMT. Individual differences in TPMT activity associated with this SNP are now used to determine appropriate dosage range and interval for treatment.

Alone, this variant is known as TPMT*3C -- but often, especially in Caucasians, it is found together with another nonsynonymous variant (A154T) to produce the TPMT*3A variant. Both variants are associated with loss of thiopurine methyltransferase (TPMT) activity, although *3C is milder than *3A. Inability to metabolize thiopurine drugs can lead to severe adverse reactions. Heterozygotes may be advised to take a reduced dosage due to reduced metabolism of the drug.